Introduction of downlink shared services, (i.e., broadcast or multicast transmissions), over a high speed physical downlink shared channel (HS-PDSCH) has been discussed in several contexts including enhanced multimedia broadcast multicast services (MBMS) and transmissions to wireless transmit/receive units (WTRUs) in a radio resource control (RRC) CELL_FACH state. For the downlink shared services, the same data stream is intended for a plurality of WTRUs that are known or thought to be in a cell, and the network may allow the data to be viewable to other WTRUs. Guarantee of data delivery to some or most of the WTRUs is important and a mechanism to provide such a guarantee should be supported.
Using HS-PDSCH or similar channel for delivery of the downlink shared services offers several advantages. The HS-PDSCH is a shared physical channel well suited for delivery of services across a wide-range of quality of service (QoS) classes. The HS-PDSCH is also optimized for packet services as most shared services are likely to be, (e.g., a forward access channel (FACH) data and MBMS data are most likely packetized). The HS-PDSCH also supports hybrid automatic repeat request (HARQ), which, if combined with an appropriate feedback mechanism, can be used to guarantee or significantly improve packet delivery.
In order to take advantage of the HARQ mechanism of HS-PDSCH, a feedback mechanism is required which allows the WTRUs to send a positive acknowledgement (ACK) or a negative acknowledgement (NACK) feedback to a Node-B. In high speed downlink packet access (HSDPA), the ACK or NACK message is delivered to the Node-B via a dedicated uplink channel, (i.e., high speed dedicated physical control channel (HS-DPCCH)). This not only guarantees availability of channel resources to deliver the ACK or NACK message, it also allows the Node-B to identify which WTRU a particular ACK or NACK message originates from.
Additionally, performance of HSDPA is significantly enhanced through the availability of channel quality indicator (CQI) feedback from the WTRUs. Conventionally, the CQI is also sent via the HS-DPCCH and the Node-B may identify the source of the CQI.
While the approach above is practical when the HS-PDSCH is primarily used to carry dedicated data in a CELL_DCH state, it is no longer practical for delivery of shared data or dedicated data when the WTRUs are operating in a CELL_FACH state. Any other currently available mechanisms for delivery of ACK/NACK and CQI feedback are insufficient for state operation outside of CELL_DCH state, (i.e., when dedicated resources are unavailable). There may be a very large number of WTRUs listening to a particular shared service in a cell. Dedicating a resource to these WTRUs and requiring ACK/NACK feedback of every single packet from these WTRUs will have a highly detrimental impact on the uplink capacity of the communication systems. Moreover, WTRUs not registered in a cell cannot have an access to the resources.
Since a dedicated resource is not allocated in a CELL_FACH state, the only currently available alternative for delivering an ACK or NACK message and a CQI is via a random access channel (RACH). Delivering an ACK or NACK message and a CQI via a RACH would likely to have a severe impact on the uplink capacity and is not practical. If the ACK or NACK messages and a CQI are delivered from all WTRUs, given that the downlink data is shared among a large number of WTRUs, conventional RACH operation may require a large number of retransmission of almost all data. Therefore, delivering feedback via a RACH is impractical.
It would be desirable to provide a mechanism for feedback from WTRUs for a downlink shared service, while the impact on the uplink and downlink capacity is minimal.